Lately, many researchers are studying Orthogonal Frequency Division Multiple Access (OFDMA) schemes and a Single Carrier Frequency Division Multiple Access (SC-FDMA) scheme in progress to develop a method for transmitting data through a radio channel at a high speed.
OFDMA is a scheme for transmitting data using multi-carrier. That is, OFDMA receives a serial symbol sequence and modulators the received serial symbol sequence to a plurality of sub-carriers having orthogonality by converting the serial symbol sequence to parallel data.
FIG. 1 is a block diagram illustrating a transmitter of an OFDMA system in accordance with the related art.
Referring to FIG. 1, the OFDMA transmitter includes an encoder 11, a modulator 12, a serial to parallel (S/P) converter 13, an N sized Inverse Fast Fourier Transform (IFFT) processor 14, a parallel to serial (P/S) converter 15, and a cyclic prefix (CP) inserter 16.
The encoder 11 performs a channel encoding process. That is, the encoder 11 receives sequences of information bits and performs the channel encoding process on the received sequences. In general, a convolutional encoder, a turbo encoder, or a low density parity check (LDPC) encoder is used as the encoder 11.
The modulator 12 performs a modulation process based on a quadrature phase shift keying scheme (QPSK), 8PSK, 16-ary quadrature amplitude modulation (16QAM), 64QAM, or 256QAM.
The S/P converter 13 receives the modulated data from the modulator 12 and converts the received data to parallel data. The IFFT processor 14 receives the parallel data from the S/P converter 13 and performs the IFFT process on the received parallel data. The P/S converter 15 converts the output from the IFFT processor 14 to serial data. The cyclic prefix inserter 16 inserts a cyclic prefix to the output data of the P/S converter 15.
The IFFT processor 14 converts frequency domain input data to time domain output data. Since input data is generally processed in a frequency domain in an OFDMA system, the OFDMA system has a disadvantage that a peak-to-average power ratio (PAPR) decreases when the IFFT processor 14 converts the frequency domain input data into the time domain output data.
The PAPR is one of critical factors to be considered to transmit data in a backward direction. If the PAPR increases, cell coverage becomes narrowed. Accordingly, a terminal requires increasing a signal power.
In order to overcome the shortcoming of the OFDMA system, a SC-FDMA scheme was introduced for uplink transmission.
FIG. 2 is a block diagram illustrating a transmitter of a SC-FDMA system in accordance with the related art.
Referring to FIG. 2, the transmitter for the SC-FDMA system includes an encoder 21, a modulator 22, a S/P converter 23, a Discrete Fourier Transform (DFT) processor 24, an IFFT processor 25, a P/S converter 26, and a cyclic prefix inserter 27.
The encoder 21 receives predetermined sequences of information bits and performs a channel encoding process thereon. The modulator 22 modulates the encoded data based on one of QPSK, 8PSK, 16QAM, 64QAM, and 256QAM schemes. The S/P converter 23 receives the modulated data from the modulator 22 and converts the modulated data to parallel data. The DFT processor 24 receives the parallel data from the S/P converter 23 and performs the DFT process thereon. The IFFT processor 25 receives the transformed data from the DFT processor 24 and performs the IFFT process thereon. Here, a mapping processor (not shown) may be disposed between the DFT processor 24 and the IFFT processor 25.
The mapping processor maps the output data of the DFT processor to the input data of the IFFT processor. That is, the mapping processor maps the output data of the DFT processor to corresponding input points of the IFFT processor to load the frequency domain data acquired from the DFT processor on sub-carriers. Here, the output symbols from the DFT processor are sequentially mapped to the input points of the IFFT processor in order to use consecutive sub-carriers on a frequency domain. Such a mapping method is referred as a localized allocation. Also, the output symbols from the DFT processor may be mapped to the input points of the IFFT processor at a predetermined interval in order to use sub-carriers separated at the same interval on a frequency domain. Such a mapping method is referred as a distribution allocation.
The P/S converter 26 converts the output data of the IFFT processor 25 to serial data. The cyclic prefix inserter 27 inserts a cyclic prefix in the output data of the P/S converter 26.
Meanwhile, a DFT function and an IDFT function may be replaced with an FFT function and an IFFT function. N may be an integer such as 1, 2, 3, and 4 for the DFT Function and the IDFT function. Also, N may be a square value of 2, such as 1, 2, 4, 8, and 16 for the FFT function and the IFFT Function.
FIG. 3 illustrates a sub-frame structure for SC-FDMA in accordance with the related art.
In the SC-FDMA scheme, a radio frame, a basic unit for backward transmission, is defined to have a length of 10 mn. A radio frame includes 20 sub-frames each having a length of 0.5 ms. One sub-frame includes six long blocks (LB) and two short blocks (SB). A cyclic prefix is inserted at the front of each block. The long block is for transmitting information except pilot data and the short block is for transmitting the pilot data only.
As described above, the SC-FDMA system has an advantage of a wide cell boundary for an uplink transmission of a terminal because the SC-FDMA system has a low PAPR. However, the SC-FDMA system has disadvantages as follows. That is, the performance of the SC-FDMA system deteriorates in high level modulation such as quadrature amplitude modulation (QAM), the SC-FDMA system is inefficient for a multi antenna system such as multiple input multiple output (MIMO) systems, the SC-FDMA system has a low flexibility of managing resources because of the necessity of a predetermined sub-carrier allocation method, and the SC-FDMA system has a difficulty in using various pilot patterns. Therefore, the SC-FDMA scheme is better for a wide cell boundary where a terminal requires increasing transmission power to transmit data. Otherwise, the OFDMA scheme is better. Accordingly, there is a demand for developing a method for selectively and adaptively using the SC-FDMA scheme and the OFDMA scheme in a terminal according to an environment in order to provide the optimal performance.